Deleting act from Magna Carta involved reordering ... (7)
I believe the answer is:
anagram
'reordering' is the definition.
(I've seen this before)
'deleting act from magna carta involved' is the wordplay.
I cannot quite see how this works, but
'act' could be 'am' (AM is an example) and 'am' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for anagram that I've seen before include "Letter puzzle" , "Word rearranging another's letters" , "Cruciverbal jumble" , "Rearrangement of a word's letters to form new word(s)" , "reordering" .)